SLPVectorizer: Improve the cost model for loop invariant broadcast values.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179930 91177308-0d34-0410-b5e6-96231b3b80d8
Fix an unused variable warning.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179931 91177308-0d34-0410-b5e6-96231b3b80d8